Fast-charging voltage generator

ABSTRACT

A voltage generator includes an oscillator, a charge pump, a smoothing capacitor, and a driving controller. The oscillator has an output. The charge pump has an input and an output, and the input of the charge pump is coupled to the output of the oscillator. The smoothing capacitor is coupled to the output of the charge pump. The driving controller is coupled to the oscillator, and generates an enable signal to adjust an operation frequency of the oscillator. The voltage generator supplies a driving voltage to a switch for driving the switch via the smoothing capacitor. The driving controller generates the enable signal according to the driving voltage.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of the prior application toChen et al., application Ser. No. 15/398,726, filed Jan. 5, 2017, whichis a continuation of the prior application to Chen et al., applicationSer. No. 14/886,343, filed Oct. 19, 2015, which claims the benefit ofU.S. Provisional Application No. 62/089,907, filed Dec. 10, 2014.

TECHNICAL FIELD

The present disclosure relates to voltage generators and, morespecifically, to fast-charging voltage generators that may be employedto support associated circuitry.

BACKGROUND

Radio frequency (RF) switches are important building blocks in manywired and wireless communication systems. Solid state RF switches arefound in many different communication devices such as cellulartelephones, wireless pagers, wireless infrastructure equipment,satellite communications equipment, and cable television equipment. Asis well known, the performance of a solid state RF switch may becharacterized by one of any number of operating performance parametersincluding insertion loss and switch isolation. Performance parametersare often tightly coupled, and any one parameter can be emphasized inthe design of RF switch components at the expense of others. Othercharacteristics that are important in RF switch design include ease anddegree (or level) of integration of the RF switch, complexity, yield,return loss and, of course, cost of manufacture.

Still other performance characteristics associated with RF switches ispower handling capability and switching speed. When the power handlingcapability of an RF switch is low, the RF switch might not be able toisolate one path from another if the input signal is too great. That is,the peak-to-peak voltage swing of an input signal might be sufficientlyhigh to overcome the reverse bias of a given transistor or transistorgroup, thus effectively causing such a transistor or transistor groupthat has been placed in an OFF state (reverse bias state) to be in anundesirable ON state, and effectively ruin the switching capability ofthe RF switch. Switching speed is closely related to power handling inthat if the speed of switching is not fast enough, a given switch pathmight not be isolated quickly enough and thus portions of received ortransmitted signals might undesirably be present on selected branches ofthe switch.

SUMMARY

One embodiment of present invention discloses a voltage generator. Thevoltage generator includes an oscillator, a charge pump, a smoothingcapacitor, and a driving controller.

The oscillator has an output. The charge pump has an input and anoutput, and the input of the charge pump is coupled to the output of theoscillator. The smoothing capacitor is coupled to the output of thecharge pump. The driving controller is coupled to the oscillator, andgenerates an enable signal to adjust an operation frequency of theoscillator.

The voltage generator supplies a driving voltage to a switch for drivingthe switch via the smoothing capacitor. The driving controller generatesthe enable signal according to the driving voltage.

Another embodiment of present invention discloses a voltage generator.The voltage generator includes an oscillator, a charge pump, a smoothingcapacitor, and a driving controller.

The oscillator has an output. The charge pump has an input and anoutput, and the input of the charge pump is coupled to the output of theoscillator. The smoothing capacitor is coupled to the output of thecharge pump. The driving controller is coupled to the oscillator, andgenerates an enable signal to adjust an operation frequency of theoscillator.

The voltage generator supplies a driving voltage to a RF switch fordriving the switch via the smoothing capacitor. During a switchingperiod of the RF switch, the driving controller generates the enablesignal according to the driving voltage so that the operation frequencyof the oscillator is controlled to be faster.

Another embodiment of present invention discloses a voltage generator.The voltage generator includes an oscillator, a charge pump, a smoothingcapacitor, and a driving controller.

The oscillator has an output. The charge pump has an input and anoutput, and the input of the charge pump is coupled to the output of theoscillator. The smoothing capacitor is coupled to the output of thecharge pump. The driving controller is coupled to the oscillator, andgenerates an enable signal to adjust an operation frequency of theoscillator.

The voltage generator supplies a driving voltage to a circuit via thesmoothing capacitor. The driving controller generates the enable signalaccording to the driving voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more readily apparent to thoseordinarily skilled in the art after reviewing the following detaileddescription and accompanying drawings, in which:

FIG. 1A depicts a block diagram of a voltage generator in accordancewith a first embodiment of the present disclosure.

FIG. 1B depicts a block diagram of a voltage generator in accordancewith a second embodiment of the present disclosure.

FIG. 2 depicts more details of several components shown in FIGS. 1A and1B in accordance with an embodiment of the present disclosure.

FIG. 3 is a schematic diagram of another embodiment of a regulatedvoltage generator of a ring oscillator in accordance with an embodimentof the present disclosure.

FIG. 4 is a schematic diagram of an inverter component of a ringoscillator in accordance with an embodiment of the present disclosure.

FIGS. 5A-5C are schematic diagrams including a switchable shortingelement in accordance with embodiments of the present disclosure.

FIGS. 6A-6D are graphs showing the effects of the application of anenable signal in accordance with an embodiment of the presentdisclosure.

FIGS. 7A and 7B are graphs showing pre-charging and normal operationstates corresponding to fast and slow voltage charging periods inaccordance with an embodiment of the present disclosure.

FIG. 8 depicts control logic that is employed to operate an RF switchlike that shown in shown in FIG. 9 in accordance with an embodiment ofthe present disclosure.

FIG. 9 depicts a radio frequency (RF) switch that can be controlled tooperate in accordance with the fast and slow charging periods inaccordance with an embodiment of the present disclosure.

FIG. 10 depicts components of a voltage generator, with an emphasis onthe application of the enable signal on a regulated voltage generatorand the switchable shorting element in accordance with an embodiment ofthe present disclosure.

FIGS. 11A-C and FIGS. 12A-C are graphs of a voltage generator outputvoltage under different states of switches shown in FIG. 10 inaccordance with an embodiment of the present disclosure.

FIG. 13 depicts a voltage generator according to another embodiment ofthe present invention.

FIG. 14 depicts a driving controller according to another embodiment ofthe present invention.

FIG. 15 shows the waveforms of the enable signal and the driving voltageaccording to the driving controller in FIG. 14 in one embodiment of thepresent invention.

DETAILED DESCRIPTION

Below, exemplary embodiments will be described in detail with referenceto accompanying drawings so as to be easily realized by a person havingordinary knowledge in the art. The inventive concept maybe embodied invarious forms without being limited to the exemplary embodiments setforth herein. Descriptions of well-known parts are omitted for clarity,and like reference numerals refer to like elements throughout.

FIG. 1A depicts a block diagram of a voltage generator 100 in accordancewith a first embodiment of the present disclosure. As shown, voltagegenerator 100 includes a rising/falling edge trigger 110 that isresponsive to control signals CS1 . . . CSn, and that generates anenable signal EN according to a rising or falling edge of the controlsignals CS1 . . . CSn. As will be explained below with respect to FIGS.8 and 9, control signals CS1 . . . CSn may be used as control signals toenable or disable respective branches of a radio frequency (RF) switch.As those skilled in the art will come to appreciate, the embodimentsdescribed herein improve the switching performance of such an RF switch.

As further shown in FIG. 1A, voltage generator 100 includes anoscillator 130, such as a ring oscillator, a clock buffer 135, a chargepump 140, a switchable shorting element (e.g., a transistor) 150, afirst resistor R1 (i.e., an additional resistor) and a second resistorR2 (i.e., a resistor) connected in series and connected between anoutput of the charge pump 140 and a smoothing capacitor C1. The node 145connecting second resistor R2 and one side of smoothing capacitor C1 isdesignated as Vout and is the output that supplies a desired voltage togates of respective transistors that control whether a given path of anRF switch is enabled or disabled (as will be described more fully withrespect to FIGS. 8 and 9). Vout could be a positive voltage (VPOS) or anegative voltage (VNEG). In an alternative embodiment, clock buffer 135could be replaced by a rail-to-rail amplifier, or be eliminated (shownin FIG. 1B). Switchable shorting element 150 is connected in parallelwith second resistor R2 and which, when turned on, causes secondresistor R2 to be bypassed or shorted, thereby reducing the RC constantmade up of first and second resistors R1, R2 and smoothing capacitor C1.Further operational details of switchable shorting element 150 aredescribed later herein.

Ring oscillator 130 is comprised of a regulated voltage generator 155that generates regulated voltage Vreg, a bias current generator 160 thatgenerates a bias current Iref, and an inverter module 170 made up of aplurality of inverters 171(a)-171(n).

Each of the inverters 171(a)-171(n) is powered by a respectivetransistor 172(1)-172(n) with regulated voltage generator 155 and biascurrent generator 160 providing the desired voltage and current thereto.Gate voltage for transistors 172(1)-172(n) is generated by Iref andtransistor 162, which is operated in its saturation region. Thus, thegate voltage equals Vreg-VGS (for transistor 162).

As further shown, enable signal EN is supplied to switchable shortingelement 150 and to oscillator 130, and thus also to regulated voltagegenerator 155, bias current generator 160 and inverter module 170. Theresult of the application of the enable signal will be described infurther detail below. In one embodiment, rising falling edge trigger 110generates enable signal EN for a predetermined period of time, e.g.,approximately 2 μsec, by use of, e.g. a delayer circuit (shown in FIG.8). That is, enable signal EN may be considered a momentary signal forpurposes of assisting an RF switch to change from a start up mode to anormal operational mode, or to change from one mode to another modeduring normal operations.

FIG. 1B depicts a block diagram of a voltage generator 101 in accordancewith a second embodiment of the present disclosure. As shown, voltagegenerator 101 includes rising/falling edge trigger 110 that isresponsive to control signals CS1 and CS2, and that generates enablesignal EN according to a rising or falling edge of the control signalsCS1, CS2. In this embodiment, the input of charge pump 140 is coupled tothe output of the oscillator 130 (i.e., without intervening clock buffer135 as shown in FIG. 1A). Other than the removal of clock buffer 135,the main difference between voltage generator 100 in FIG. 1A and voltagegenerator 101 in FIG. 1B is that inverter module 170 comprises at leasttwo inverters 171(a) and 171(b). As will be explained below, enablesignal EN controls the sizing of inverter module 170, i.e., enablesignal EN is employed to select transistors with different gate sizes,thereby influencing the frequency of oscillator 130.

A goal of embodiments of the present disclosure is to quickly chargesmoothing capacitor C1 to a desired voltage, which may then be availablefor use to, e.g., forward or reverse bias a switching element in anothercomponent of an overall circuit (e.g., as shown in FIG. 9). That is, itmay be desirable to have a higher (or lower)-than-nominal voltageavailable for a variety of purposes, and sometimes it is particularlyhelpful to be able to generate such a voltage very quickly.

More specifically, in accordance with embodiments of the presentdisclosure, voltage generator 101 (as an example) operates in twostates. In a first, fast or pre-charging, state, the voltage generatoroperates to quickly charge smoothing capacitor C1 to an intermediatevoltage that is sufficiently high or low for selected applications, andin a second, slow charging, state, the voltage generator operates toreduce noise and to further charge the smoothing capacitor to a lowernegative voltage when Vout is a negative voltage (VNEG), or to a higherpositive voltage when Vout is a positive voltage (VPOS).

In one embodiment, as shown in FIGS. 7A and 7B, in the first, fastcharging, state, smoothing capacitor C1 is charged to V2 (e.g., 0.9*V3)from V1 in 2 microseconds (the period of enable signal EN), and in thesecond, slow charging, state, smoothing capacitor C1 is charged furtherto V3.

To achieve the different states, the enable signal EN is selectivelygenerated/applied when the first, fast charging, state is desired. Thatstate, as noted, may last on the order of 2 microseconds. The defaultstate, maybe considered the second, slow charging, state.

The generated enable signal EN may be applied to one or more of severalcomponents that enable voltage generator 101 (or 100) to generate thedesired voltage within the desired timeframe.

FIG. 2 depicts more details of the several components shown in FIG. 1Bin accordance with an embodiment of the present disclosure. Thoseskilled in the art will appreciate that the details shown in FIG. 2 areexemplary only, and are not meant to be limiting in any way. Otherimplementations are possible as those skilled in the art willappreciate.

In FIG. 2, regulated voltage generator 155 comprises series connectedresistors R9, R10 and R11 that provide a path to ground for VDD viatransistor M202. A reference voltage Vref is used as one input to anopamp 210 whose other input is supplied from the node between resistorsR9 and R10. Transistor M204 controls whether resistor R10 is bypassed ornot. That is, upon the application of enable signal EN via inverter 205,resistor R10 is placed back in series with R9 and R11 thereby increasingthe voltage of Vreg.

As noted, embodiments of the present invention provide a fast chargingstate and a slow charging state. In the fast charging state (orpre-charging state):

EN′ (invert of EN)=Low

Vreg=Vref×[1+(R11+R10)/R9]

In the slow charging state:

EN′ (invert of EN)=High

Vreg=Vref×[1+(R11/R9)]

Vreg (Pre-charging state, Vreg1)>Vreg (Normal operation state, Vreg2)

A higher Vreg results in a higher frequency for oscillator 130.

FIG. 3 depicts another possible embodiment for regulated voltagegenerator 155. In this case, a non-inverted version of enable signal ENis employed, and an input to op-amp 210 is from a node between R10 andR11.

Thus, in the embodiment of FIG. 3, in the fast chargingstate/Pre-charging state:

EN=High

Vreg=Vref×[1+[R11/R9]

In the slow charging/normal operation state:

EN=Low

Vreg=Vref×[1+(R11/(R9+R10))]

Thus, and referring again to FIG. 2, when the enable signal EN is high,inverter 205 inverts the high enable signal EN and turns off M204. This,in turn, places resistor R10 in the circuit resulting in an increasedVreg regulated voltage. This regulated voltage is supplied to the biascurrent generator 160 and to the several inverters 171(1), etc. of ringoscillator 130. By increasing the regulated voltage, the frequency ofring oscillator 130 increases, thereby more quickly achieving the fastcharge over the course of, e.g., the predetermined 2 microsecond fastcharging period.

Still referring to FIG. 2, bias current generator 160 is comprised oftransistor M206 and current sources 230 and 235. Voltage Vreg issupplied through transistor M208. Enable signal EN is used to controltransistor M206.

In this configuration of bias current generator 160, in the fastcharging state/Pre-charging state:

EN=High, M206 is turn ON

I1=Iref+Iref1, I2 (to inverter)=I1

In the slow charging state/Normal operation state:

EN=Low, M206 is turned Off

I1=Iref, I2 (to inverter)=I1

Thus, as shown, in the first state, the high enable signal EN issupplied to transistor M206, which adds an additional bias currentdesignated as Iref1 to the circuit. The higher bias current resultingfrom the application of the high enable signal EN causes an increase ofthe frequency of ring oscillator 130, which results in the desired fastcharging state.

Referring back to FIG. 1B and still to FIG. 2, inverter module 170 iscomprised of, in this example embodiment, inverters INV1 and INV2.Inverter INV1 is comprised of inverters 250, 255, transistor M210 viawhich Vreg is supplied, and transistors M220 and M222. Inverter INV2 iscomprised of inverters 260, 265, transistor M212 via which Vreg issupplied, and transistors M224 and M226.

FIG. 4 is a more detailed depiction of inverter module 170 and furthershows how INV1, for example, is comprised of INV11 comprising transistorM405 and INV12 comprising transistor M410. As shown, enable signal EN issupplied to transistor M222 and an inverted form of enable signal EN issupplied to transistor M220. In the first state, enable signal EN ishigh. Thus, transistor M222 is turned on and transistor M220 is turnedoff so that transistor M410 is active and transistor M405 is inactive.In this configuration, a first-second inverter INV12, formed by at leastone PMOS transistor M210 and at least one NMOS transistor M410, isenabled for oscillator 130 operation. In one embodiment, the effectivegate length L of the first inverter INV1 is resized to 0.18 μm, i.e.,the gate length of M410.

In the second state, i.e., the slow charging state, the enable signal ENis low. Thus, transistor M222 is turned off and transistor M220 isturned on so that transistor M410 is inactive and transistor M405 isactive. In this configuration, a first-first inverter INV11, formed byat least one PMOS transistor M210 and at least one NMOS transistor M405,is enabled for oscillator 130 operation. In one embodiment, theeffective gate length L of the first inverter INV1 is resized to 5 μm,i.e., the gate length of M405. With a smaller gate length, the frequencyof the ring oscillator 130 is increased thereby achieving the desiredfast charging when the enable signal EN is high. In another embodiment,NMOS transistor M405 could be designed with a smaller gate width andNMOS transistor M410 could be designed with larger gate width.

Reference is now made to FIG. 5A, which is a schematic of switchableshorting element 150 used in voltage generator 100 or 101 in accordancewith an embodiment of the present disclosure. As shown, switchableshorting element 150 comprises transistor M505. When a high enablesignal EN is applied to transistor M505 it is turned on, thus shortingthe second resistor R2. In the second, slow charging, state, transistorM505 is turned off thus forcing resistor R2 back into the circuit, andincreasing the RC constant in connection with the smoothing capacitorC1.

Thus, in the fast charging state/Pre-charging state:

Vout=Vin×(1−e ^(−t/R1C))

In the slow charging state/Normal operation state:

Vout=Vin×[1−e ^(−t/(R1+R2)C)]

FIG. 5B shows a different configuration wherein resistor R1 is coupledbetween a node, defined by a connection between the resistor R2 andcharge pump 140, and switchable shorting element 150. The configurationof FIG. 5C is similar to that shown in FIG. 5A, but resistor R1 iseliminated.

FIGS. 6A and 6B depict, respectively, graphs of Vout if enable signal ENis kept high indefinitely, or is never applied at all.

FIG. 6C shows the application of enable signal EN for approximately 2μsec, and FIG. 6D shows that the desired Vout is quickly obtained, andbecomes stable almost immediately compared to the graph of FIG. 6A.

FIGS. 7A and 7B are graphs showing fast, pre-charging (first state) andnormal operation (second state) states corresponding to fast and slowvoltage charging periods in accordance with an embodiment of the presentdisclosure.

The graph of FIG. 7A depicts an instance when, e.g., an electronicdevice is first powered on and an RF switch, for example, will be usedfor the first time. Such an instance may be considered a start-up orpre-charging state. The graph of FIG. 7B depicts an instance when, afterthe electronic device is in operation, there is a need to change whichpath of an RF switch is enabled.

In FIGS. 7A and 7B “fp” connotes the pre-charging or mode-to-modefrequency of oscillator 130, and “fn” connotes the normal frequency,where fp>fn. The indicated ts and td markings denote the beginning andend times of enable signal EN.

FIG. 8 depicts control logic that is employed to operate an RF switchlike that shown in shown in FIG. 9. FIG. 8 depicts rising/falling edgetrigger 110, which receives as inputs control signals CS1 and CS2, adelayer 112, a level shifter 114, oscillator 130, clock buffer 135,charge pump 140 and switchable shorting element 150, along withresistors R1 and R2. Delayer 112 operates to ensure that enable signalEN is kept high for the desired 2 μsec, or other desired period of timeto effect to fast charging mode as described herein. Level shifter 114may be employed to adjust the applied voltage to a gate terminal of thetransistor that may be employed for switchable shorting element 150.

As can be seen in FIG. 8 Vreg and Vout are supplied to pass transistorlogic circuits 802, 804, 806 and 808. These logic circuits areresponsible for supplying as outputs, either Vreg or Vout. Outputs aresupplied as Vctrl1, Vctrl2, Vctrl3, and Vctrl4, which are supplied togates of transistors in RF switch 900 of FIG. 9 having an RF common(RFC) terminal, and two input/output terminals RF1, RF2.

Which voltage, Vreg or Vout is applied to respective Vctrl1, Vctrl2,Vctrl3, and Vctrl4 outputs is dictated by logic circuit 800 using asinputs CS1 and CS2, the same inputs that are monitored for rising andfalling edges to trigger enable signal EN generation. In other words, asthe states of CS1 and CS2 change, that change will be detected byrising/falling edge trigger 110 causing enable signal EN to be appliedfor, e.g., the 2 μsec period. At the same time, logic circuit 800 willdetermine which voltage, Vreg or Vout, should be applied to Vctrl1,Vctrl2, Vctrl3, and Vctrl4, and thus to gates of respective transistors,as this will determine which paths of RF switch 900 will be enabled.That is, Vctrl1 dictates whether the path between RF1 and RFC isenabled, Vctrl2 dictates whether the path between RF2 and RFC isenabled, Vctrl3 dictates whether a shunt path associated with RF1 isenabled, and Vctrl4 dictates whether a shunt path associated with RF2 isenabled.

By applying the enhanced Vout (i.e., lower negative or higher positivevoltage compared to Vreg) for a selected amount of time via Vctrl1,Vctrl2, Vctrl3, and Vctrl4, it is possible to more quickly cause RFswitch 900 to enable or disable paths.

Those skilled in the art will appreciate that while the embodiments ofthe voltage generator 100, 101 have been described in connection with anRF switch, voltage generator 100, 101 can also be used in connectionwith a power amplifier, a low noise amplifier, a transceiver, a PLL or afrequency synthesizer, among other possible circuits, devices orcomponents.

FIG. 10 depicts components of voltage generator 100, 101, with anemphasis on the application of enable signal EN on regulated voltagegenerator 155 and switchable shorting element 150. As indicated in thefigure, with SW1 (M204) off, i.e., with the application of enable signalEN, Vreg is higher, than when SW1 (M204) is on. SW2 (M505) also has animpact on the operation of voltage generator 100, 101.

These impacts are depicted in the graphs of FIGS. 11A-C and FIGS. 12A-C.

Specifically, FIG. 11A is a plot of Vout when SW1 and SW2 are bothalways on.

FIG. 11B is a plot of Vout when SW1 is always on, and SW2 is always off.

FIG. 11C shows a plot when SW1 is always on, and SW2 is on only for 2μsec, i.e., the time period of enable signal EN.

FIG. 12A is a plot of when SW1 is always off and SW2 is always on.

FIG. 12B is a plot of when SW1 is always off, and SW2 is always off.

FIG. 12C is a plot of SW1 being off only for approximately 0.3 μsec andSW2 is on only for approximately 0.3 μsec.

FIG. 13 shows a voltage generator 200 according to another embodiment ofthe present invention. In some embodiments, the voltage generator 200can supply the driving voltage Vout to a circuit CKT1.

The voltage generators 100 and 200 have similar structures and can beoperated with similar principles; however, the voltage generator 200includes a driving controller 210. The driving controller 210 is coupledto the oscillator 130, and can generate the enable signal EN to adjustthe operation frequency of the oscillator 130. For example, the drivingcontroller 210 can control the operation frequency of the oscillator 130to be faster during a transition period of the circuit CKT1.

In some embodiments, the circuit CKT1 can be an RF switch, such as theRF switch 900 shown in FIG. 9, and the driving voltage Vout can be usedfor driving the switch via the smoothing capacitor C1. Also, theoscillator 130 can have the same structures as shown in FIGS. 1A, FIG.1B, and FIG. 2.

In some embodiments, the driving controller 210 can generate the enablesignal EN according to the driving voltage Vout. For example, when thedriving voltage Vout is within a predetermined region (e.q. 2V>Vout>0V),the driving controller 210 will issue the enable signal EN to cause theoscillator 130 to operate at a first frequency. However, when thedriving voltage Vout is beyond the predetermined region (e.q. Vout>2),the driving controller 210 will stop issuing the enable signal EN tocause the oscillator 130 to operate at a second frequency lower than thefirst frequency.

That is, when the driving voltage Vout is within the predeterminedregion, the driving controller 210 can control the oscillator 130 tooperate at a higher frequency so that the driving voltage Vout can reachthe desired level (e.q. 3V) faster.

In some embodiments, the voltage generator 200 can also include theresistor R2 and the shorting element 150 as the voltage generator 100,and the driving controller 210 can also turn on the shorting element 150when the driving voltage Vout is within the predetermined region,thereby adjusting the RC constant and allowing the driving voltage Voutto reach the desired level even faster. Similarly, the resistor R1 shownin FIGS. 1A and 5B may also be added to the voltage generator 200.

Furthermore, the smoothing capacitor C1 can be coupled between thesecond terminal of the resistor R2 and a reference terminal. Thereference terminal can be, for example, but not limited to, the voltageterminal providing the ground voltage.

In FIG. 13, the driving voltage Vout is a positive voltage. In thiscase, the driving controller 210 can include a comparator 212 forgenerating the enable signal EN by comparing the driving voltage Voutwith a reference voltage VB1. The comparator 212 can issue the enablesignal EN of a high voltage when the driving voltage Vout is smallerthan the reference voltage VB1. In some embodiments, the comparator 212can be a hysteresis comparator so that the enable signal EN could beglitch-free.

However, in some embodiments, the RF switch may be driven by a negativevoltage. In this case, for example, when the driving voltage Vout iswithin a predetermined region (e.q. 0V>Vout>−2V), the enable signal ENmay be issued to cause the oscillator 130 to operate at a higherfrequency. However, when the driving voltage Vout is beyond thepredetermined region (e.q. −2V>Vout), the oscillator 130 can becontrolled to operate at a lower frequency.

Also, since the driving voltage Vout is a negative voltage, the drivingcontroller may require a voltage combiner to adjust the comparisonvoltage for the comparator. FIG. 14 shows a driving controller 310according to another embodiment of the present invention.

The driving controller 310 includes a comparator 312 and a voltagecombiner 314. The voltage combiner 314 can receive the driving voltageVout and shift the driving voltage Vout according to the combiningvoltage VR to generate a positive comparing voltage VP1. The comparator312 is coupled to the voltage combiner 314, and can generate the enablesignal EN by comparing the positive comparing voltage VP1 with areference voltage VB2. That is, with the combiner 314, the comparator312 can compare two positive voltages, thereby simplifying the design ofthe comparator 312.

In FIG. 14, the voltage combiner 314 includes a transistor M1, andvoltage division resistors RD1 and RD2. The transistor M1 has a firstterminal for receiving the supply voltage VC, a second terminal, and acontrol terminal for receiving the combining voltage VR. The combiningvoltage VR is a positive reference voltage for shifting the drivingvoltage Vout from negative to positive.

The voltage division resistor RD1 has a first terminal coupled to thesecond terminal of the transistor M1, and a second terminal foroutputting the positive comparing voltage VP1. The second voltagedivision resistor RD2 has a first terminal coupled to the secondterminal of the first voltage division resistor RD1, and a secondterminal for receiving the driving voltage Vout. By selecting theresistance values of the voltage division resistors RD1 and RD2properly, the voltage division resistors RD1 and RD2 can provide thepositive comparing voltage VP1 between the combining voltage VR and thenegative driving voltage Vout. The positive comparing voltage VP1 couldbe expressed by as following.

${{VP}\; 1} = {{\frac{\left( {{VR} - {VGS} - {Vout}} \right)}{\left( {{{RD}\; 1} + {{RD}\; 2}} \right)} \times {RD}\; 2} + {Vout}}$

Wherein VGS is the gate-source threshold voltage of the transistor M1.

In FIG. 14, the transistor Ml can be used to buffer the combiningvoltage VR so that the driving voltage Vout could have better drivingcapability. However in some other embodiments, the transistor M1 can beomitted, and the first terminal of the voltage division resistor RD1 canreceive the combining voltage VR directly according to the systemrequirement.

Furthermore, the comparator 312 shown in FIG. 14 is a hysteresiscomparator; however, in some other embodiments, the comparator 312 mayalso be implemented by comparators with other structures or of differenttypes.

FIG. 15 shows the waveforms of the enable signal EN and the drivingvoltage Vout according to the driving controller 310 in one embodimentof the present invention. In FIG. 15, the target level of the drivingvoltage Vout is −3V. In this case, during the period P1, before thedriving voltage Vout reaches the predetermined level, for example, −2V,the driving controller 310 can generate the enable signal EN at a highvoltage level to increase the charging speed of the driving voltageVout. However, during the period P2, after the driving voltage Voutreaches the predetermined level (−2V), the driving controller 310 willstop generating the high voltage enable signal EN. Therefore, theoperation frequency of the oscillator 130 can be decreased, and thecharging speed of the driving voltage Vout will also slow down.Consequently, the driving voltage Vout can reach the target level (−3V)smoothly, and the power consumption and the noise came from theoscillator 130 could be reduced.

In summary, the voltage generator provided by the embodiments of thepresent invention can provide the driving voltage to the externalcircuit, and adjust the operation frequency of the oscillator to improvethe switching performance during the transition of the external circuitand/or according to the level of the driving voltage.

While the disclosure has been described in terms of what is presentlyconsidered to be the most practical and preferred embodiments, it is tobe understood that the disclosure need not to be limited to the aboveembodiments. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A voltage generator, comprising: an oscillatorhaving an output; a charge pump having an input and an output, the inputof the charge pump being coupled to the output of the oscillator; asmoothing capacitor coupled to the output of the charge pump; and adriving controller coupled to the oscillator, and configured to generatean enable signal to adjust an operation frequency of the oscillator;wherein: the voltage generator is configured to supply a driving voltageto a switch for driving the switch via the smoothing capacitor; and thedriving controller generates the enable signal according to the drivingvoltage.
 2. The voltage generator of claim 1, wherein: when the drivingvoltage is within a predetermined region, the driving controller issuesthe enable signal to cause the oscillator to operate at a firstfrequency; and when the driving voltage is beyond the predeterminedregion, the driving controller stops issuing the enable signal to causethe oscillator to operate at a second frequency lower than the firstfrequency.
 3. The voltage generator of claim 2, further comprising: aresistor having a first terminal coupled to the charge pump and a secondterminal coupled to the smoothing capacitor; and a shorting elementcoupled in parallel with the resistor, and configured to cause theresistor to be at least partially bypassed when turned on; wherein thedriving controller is further configured to turn on the shorting elementwhen the driving voltage is within the predetermined region.
 4. Thevoltage generator of claim 3, further comprising an additional resistorcoupled between the charge pump and the resistor.
 5. The voltagegenerator of claim 3, further comprising an additional resistor coupledbetween the output of the charge pump and the shorting element.
 6. Thevoltage generator of claim 3, wherein the smoothing capacitor is coupledbetween the second terminal of the resistor and a reference terminal. 7.The voltage generator of claim 2, wherein the driving voltage is anegative voltage, and the driving controller comprises: a voltagecombiner configured to receive the driving voltage and shift the drivingvoltage according to a combining voltage to generate a positivecomparing voltage; and a comparator coupled to the voltage combiner, andconfigured to generate the enable signal by comparing the positivecomparing voltage with a reference voltage.
 8. The voltage generator ofclaim 7, wherein the voltage combiner comprises: a first transistorhaving a first terminal configured to receive a supply voltage, a secondterminal, and a control terminal configured to receive the combiningvoltage; a first voltage division resistor having a first terminalcoupled to the second terminal of the first transistor, and a secondterminal configured to output the positive comparing voltage; and asecond voltage division resistor having a first terminal coupled to thesecond terminal of the first voltage division resistor, and a secondterminal configured to receive the driving voltage.
 9. The voltagegenerator of claim 7, wherein the voltage combiner comprises: a firstvoltage division resistor having a first terminal configured to receivethe combining voltage, and a second terminal configured to output thepositive comparing voltage; and a second voltage division resistorhaving a first terminal coupled to the second terminal of the firstvoltage division resistor, and a second terminal configured to receivethe driving voltage.
 10. The voltage generator of claim 7, wherein thecomparator is a hysteresis comparator.
 11. The voltage generator ofclaim 2, wherein the driving voltage is a positive voltage, and thedriving controller comprises: a comparator configured to generate theenable signal by comparing the driving voltage with a reference voltage.12. The voltage generator of claim 11, wherein the comparator is ahysteresis comparator.
 13. The voltage generator of claim 1, wherein:the oscillator comprises a second transistor and a third transistor; thesecond transistor is turned on for enabling a first inverter of theoscillator in a first state, and the third transistor is turned on forenabling a second inverter of the oscillator in a second state; thefirst state is enabled by the enable signal; and the smoothing capacitoris charged more quickly in the first state than in the second state. 14.The voltage generator of claim 13, wherein a size of the first inverteris smaller than a size of the second inverter.
 15. The voltage generatorof claim 1, wherein the switch comprises a radio frequency (RF) switch.16. A voltage generator, comprising: an oscillator having an output; acharge pump having an input and an output, the input of the charge pumpbeing coupled to the output of the oscillator; a smoothing capacitorcoupled to the output of the charge pump; and a driving controllercoupled to the oscillator, and configured to generate an enable signalto adjust an operation frequency of the oscillator; wherein: the voltagegenerator is configured to supply a driving voltage to a RF switch fordriving the switch via the smoothing capacitor; and during a switchingperiod of the RF switch, the driving controller generates the enablesignal according to the driving voltage so that the operation frequencyof the oscillator is controlled to be faster.
 17. The voltage generatorof claim 16, wherein the driving voltage is a negative voltage, and thedriving controller comprises: a voltage combiner configured to receivethe driving voltage and shift the driving voltage according to acombining voltage to generate a positive comparing voltage; and acomparator coupled to the voltage combiner, and configured to generatethe enable signal by comparing the positive comparing voltage with areference voltage.
 18. A voltage generator, comprising: an oscillatorhaving an output; a charge pump having an input and an output, the inputof the charge pump being coupled to the output of the oscillator; asmoothing capacitor coupled to the output of the charge pump; and adriving controller coupled to the oscillator, and configured to generatean enable signal to adjust an operation frequency of the oscillator;wherein: the voltage generator is configured to supply a driving voltageto a circuit via the smoothing capacitor; and the driving controllergenerates the enable signal according to the driving voltage.
 19. Thevoltage generator of claim 18, wherein the driving controller generatesthe enable signal according to the driving voltage during a transitionperiod of the circuit so that the operation frequency of the oscillatoris controlled to be faster.
 20. The voltage generator of claim 18,wherein the driving voltage is a negative voltage, and the drivingcontroller comprises: a voltage combiner configured to receive thedriving voltage and shift the driving voltage according to a combiningvoltage to generate a positive comparing voltage; and a comparatorcoupled to the voltage combiner, and configured to generate the enablesignal by comparing the positive comparing voltage with a referencevoltage.